A lithographic process is one that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Stepping and/or scanning movements can be involved, to repeat the pattern at successive target portions across the substrate. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay (the accuracy of alignment between patterns formed in different patterning steps, for example between two layers in a device) and defocus of the lithographic apparatus. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
Methods and apparatus for determining structure parameters are, for example, disclosed in WO 20120126718. Methods and scatterometers are also disclosed in US20110027704A1, US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates an illumination spot that is smaller than the grating (i.e., the grating is underfilled). In addition to scatterometry to determine parameters of a structure made in one patterning step, the methods and apparatus can be applied to perform diffraction-based overlay measurements.
Diffraction-based overlay metrology using dark-field image detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple targets can be measured in one image. Examples of dark-field imaging metrology can be found in international patent applications US2010328655 A1 and US2011069292 A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US20120044470A US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. The above documents generally describe measurement of overlay though measurement of asymmetry of targets. Methods of measuring dose and focus of a lithographic apparatus using asymmetry measurements are disclosed in documents WO2014082938 A1 and US2014/0139814A1, respectively. The contents of all the mentioned applications are also incorporated herein by reference.
However, although the invention is described in the context of an inspection apparatus, the invention is not limited in application to any particular type of inspection apparatus, or even to inspection apparatuses generally. The invention is in principle applicable to any apparatus wherein periodic measurement targets are utilized. Exemplary applications include, without limitation: measuring overlay in an inspection apparatus; focus control; measurement of critical dimension (CD); or measuring the form of a target (such as SWA or bottom tilt).
A common problem in inspection apparatuses and other optical systems is one of measurement accuracy, which is in part determined by the accuracy of the images obtained during the measurement. The image accuracy is in turn dependent on the optical system used to obtain the images. A major source of inaccuracy in images obtained from a given optical system, such as an inspection apparatus, is optical noise, e.g. stray radiation which enters the system and hits the detectors or cameras during measurements.
Stray radiation reduces the quality, and by extension the accuracy, of the detected images, and by extension the optical measurement system. This reduces the precision of the lithographic apparatus, thereby negatively impacting the precision of components produced by the apparatus, in particular in systems where the detected radiation intensities are very low. It is, therefore, desirable to remove or reduce the amount of stray radiation in an optical system, particularly an optical measurement system in a lithographic apparatus.
One source of stray radiation in an inspection apparatus is radiation being reflected from surfaces of components that are part of the optical system, e.g. tiny imperfections on the surfaces of optical components (such as mirrors or aperture stops) as well as multiple reflections off optical surfaces such as lenses. This is particularly problematic in complex lens systems, such as used in lithographic apparatuses, which contain a large number of lenses. In such systems, even a small fraction of reflected stray radiation may compound into a significant source of stray radiation and optical noise. A further source of stray radiation is radiation reflected from interior surfaces of the apparatus that are not part of the optical system, e.g. from other optical systems used for other purposes that are also housed within the inspection apparatus. A further source of stray radiation is radiation reflected off parts of the substrate other than the measurement target itself (e.g. nearby components on the substrate). Yet a further source of stray radiation is foreign particles within the system, such as microscopic dust particles floating inside the apparatus or on an optical surface.
Stray radiation can broadly be classed into two types:
a. directional stray radiation, which is e.g. caused by radiation being reflected multiple times from various surfaces either part of the optical system or not part of the optical system. For example, stray radiation reflections from glass surfaces in the optical system, such as lenses used to shape the beams, can be considered to be directional stray radiation.
b. non-directional stray radiation, which is radiation that is randomly scattered by rough surfaces, or by foreign particles, such as dust, inside the apparatus.
Conventionally, stray radiation in an optical system is reduced by use of suitable anti-reflection coatings, as well as use of apertures and stops at appropriate places in the optical path of the system. However, anti-reflection coatings may only reduce the amount of stray radiation, rather than remove it entirely. In complex optical systems, such as lens systems used in lithographic apparatuses, the compound effect of stray radiation being reflected off each lens surface may be significant. Furthermore, as described above, aperture stops may themselves be sources of stray radiation due to small imperfections in their surfaces, for example due to imperfections in their manufacturing process or by having become damaged.